1. Field of the Invention
The present invention pertains to a device for use in material testing equipment and accessories. More specifically, the present invention pertains to a mechanism for the connection of removable jaw faces to material testing grips.
2. Description of the Prior Art
In the prior art, materials testing systems are used in a variety of industries and are generally used with some sort of accessory to provide specimen gripping or holding. Many of these tests require that the accessories have specific gripping surfaces or sizes. The size and type of gripping surface needed can vary greatly from test to test, making jaw face changes a frequent event in many labs. Therefore, removable jaw faces with various sizes and surfaces are a common commodity in the materials testing industry.
Many different embodiments of jaw face attachment have developed over the years by various manufacturers. These connection mechanisms include simple pin and aperture connections as well as rigid dovetail connections. Each mechanism has advantages and disadvantages.
Zwick has implemented the use of a rigid dovetail connection as shown in FIG. 1. This connection method allows for a repeatable location for the jaw face while allowing for ease of installation. Rotation of the jaw faces is not possible with this method rendering fixed the position of the jaw faces relative to the gripped specimen. Furthermore, a tool must be used to connect these jaw faces to their holders.
MTS has implemented an externally accessible pin and a corresponding aperture to connect the jaw faces to the grips as shown in FIG. 2. This allows for jaw face rotation but poses a potential safety issue because the protruding heads and rings of the pins also move in conjunction with the jaw faces. The pull rings used for removing the pins may also get entangled in the grips. External pins may also be ejected from the apertures if measures are not taken to retain them in the grips. It appears that this has been addressed with the addition of an O-ring to the end of the pin. Additionally, the pins are a loose part which may be easily lost.
Instron, the assignee of the present application, has implemented four jaw face to grip connection methods of interest. These method include set screw and groove; internal pin an aperture; internal pin and retaining wire; and hook and fixed pin. All of these methods allow the jaw faces to rotate. This rotation is needed to compensate for both inconsistencies in specimen thickness and elastic deformation of grip bodies during testing. The set screw and groove method as shown in FIG. 3 is a simple and inexpensive method that requires a tool for installation. The set screws provide the pivot point for jaw face rotation. Faces typically rotate slightly in all directions with this connection method.
The internal pin and aperture method as shown in FIG. 4 has been perhaps the most common jaw face connection method used to date and has been adopted by various competitors. While this is perhaps the least expensive and least complicated method, there are some drawbacks. For instance, installation often requires an experienced user or an additional person to hold and manipulate the various components. Additionally, the pins may not maintain their positional integrity during testing. A slight displacement may typically be remedied by the retraction of the holders into the grip body, but occasionally it results in the pin and jaw face falling partially or completely out of the holder. Another drawback is that the pins are small loose parts which can be easily dropped or lost.
The internal pin and retention wire method was developed by the present assignee for use on side-acting screw action grips and uses a semi-permanent pin installed in the back of a typical pin and aperture jaw face as shown in FIGS. 5A and 5B. This assembly can be snapped into the holder using one hand and without the use of any tools. To remove the jaw face, simply unscrew the holder until the retention wire opens up. The jaw face can now be easily removed. While this method allows for simple installation and predictable usage patterns, there are some drawbacks associated with it. Firstly, the retention wire and holder both require special detailed machine to get the correct feel and action. Secondly, the semi-permanent pin represents a small loose part that can get lost.
On higher capacity pneumatic side-action grips, the present assignee has implemented the use of a hook and fixed pin connection method as shown in FIG. 6. A fixed pin is pressed into the back of a specially machined jaw face and acts as an attachment point for a corresponding spring-loaded hook that resides inside of the grip. Attaching the face to the grip requires that the user insert a tool onto the grip to move the spring-loaded hook far enough to slide the face over it. When the hook retracts back into the body, the face is firmly held on the grip. This eliminates loose parts, but necessitates the use of a tool for installation and removal. This apparatus further requires unique detailed faces and holders.